The failure to repair DNA double-stranded breaks (DSBs) impacts important cellular processes and can lead to the formation of translocation driven-tumors. In order to repair DSBs, cells have evolved complex DNA damage response (DDR) networks where a family of master kinases including ATM (mutated in ataxia telangiectasia) is activated. ATM phosphorylates many proteins that accumulate near DSBs in "DNA repair foci", including p53-binding protein 1 (53BP1), a protein that localizes to stalled DNA replication forks in S phase. These chromosomal regions are known to form gaps and breaks at metaphase under conditions of replication stress. The subsequent generation of DSBs from this is posited to serve as acceptors in chromosomal translocations. We generated mice defective in 53BP1 function and showed that they are growth retarded, sensitive to IR, and immunedeficient. Importantly, we have shown that 53BP1 participates in a novel end joining process that prevents the separation of free, broken DNA ends. In particular, we demonstrated that 53BP1 operates in the "joining phase" of class switch recombination (CSR), a NHEJ-like mechanism specific to the immunoglobulin heavy chain (IgH) locus of lymphoid B cells. We have proposed that such a joining-like function operates in response to DSBs to tether free DNA ends and chromatin together through its ability to 'anchor" broken DNA ends. Significantly, in the context of p53 deficiency, 53BP1-/- animals develop B lineage tumors through clonal, IgH-derived translocations with partner chromosomes that harbor oncogenes such as c-myc. We hypothesize that 53BP1-/- derived translocations result not only in c-myc amplification but also through amplification of regions harboring novel proto-oncogene(s). How partner chromosomes are determined during the genesis of translocations remains elusive and the experimental design proposed here with 53BP1 promises to shed important light on this issue. The Specific Aims of this proposal are: 1) To analyze the nature and expression of common fragile sites in 53BP1-/- cells as well as in 53BP1-/-/p53-/--derived B lineage tumors. 2) To examine the mechanisms of B lineage tumor formation in 53BP1-/-/p53-/- mice by a) characterizing novel translocation-regions isolated from B cell lymphomas and b) examining their relationship to fragile site expression.